1. Field of the Invention
The present invention relates to a light scattering type particle detector for detecting particles, which pass through a particle detecting region, by receiving light scattered by the particles.
2. Description of the Prior Art
The S/N ratio (signal-to-noise ratio) of the electric signal output by a photoelectric converter which receives scattering light depends on the amount of shot noise in the photoelectric converter. The amount of the shot noise is proportional to the light amount incident upon the photoelectric converter.
Therefore, in a case where background light (background molecule scattering light) is large, the shot noise in the photoelectric converter is increased, and thereby the S/N ratio is deteriorated.
The background light means scattering light which occurs due to molecules of medium (for example, fluid such as air or liquid) containing particles to be measured, or due to the wall surface of a flow cell for passing fluid therethrough. For example, there are 2.7xc3x971019 air molecules per 1 ml at the atmosphere. The total amount of the light scattered by such molecules is larger than that of a 0.1 xcexcm size particle.
Therefore, the larger the particle detecting region becomes, the more difficult it becomes to detect a particle. In addition, it is required to control smaller particles in a clean room for semiconductors because the level of its cleanness is improved and the concentration of particles is reduced.
A particle size detecting apparatus described in Japanese Patent No. 2786187 is known as a light scattering type particle detector for solving the above-mentioned problems.
In this apparatus, as shown in FIG. 5, a light receiving array 101 comprising a plurality of photoelectric converters 100 in a linear alignment is used for receiving light. The scattering light Ls due to particles passing through the particle detecting region D is focused on one of the photoelectric converters 100 with the condenser lens 102. The background light incident upon the photoelectric converter 100, on which the scattering light Ls is focused, is reduced by the number of the photoelectric converters 100, and thereby the deterioration of the S/N ratio in the photoelectric converter 100, on which the scattering light Ls is focused, is prevented.
However, the apparatus as shown in FIG. 5 has the following problems:
1. When the number of the photoelectric converters 100 is increased, the S/N ratio can be improved. However, in a case where the photoelectric converters 100 are arranged in a linear, i.e., one-dimensional alignment, the finite light receiving area puts limitations on increasing the number of the photoelectric converters 100.
It is preferable that the width of the gap G between the photoelectric converters be as small as possible. However, in fact, the width of the gap G has its limits. If the number of the photoelectric converters 100 is increased in the finite light receiving area, the ratio of the gap G to the light receiving area is too large, and thereby the accurate measurement is blocked. For example, as shown in FIG. 6, there is a case where the scattering light Ls due to a particle is focused on the gap G as an image P1. In this case, it is impossible to detect the particle.
2. As also shown in FIG. 6, if there is a bit of aberration in the condenser lens 102, the scattering light Ls due to a particle may be focused on the photoelectric condensers 100 adjacent each other sandwiching the gap G as an image P2. In this case, the particle may be detected as two particles or a smaller particle than its reality.
It is therefore an object of the present invention to solve these problems found in the prior art and to provide a light scattering type particle detector in which the S/N ratio is improved.
According to the first aspect of the present invention, there is provided a light scattering type particle detector for detecting particles comprising a particle detecting region formed by irradiating a light beam on sample fluid and a light receiving means for receiving light scattered by particles which pass through the particle detecting region, wherein the light receiving means is comprised of a plurality of photoelectric converters, and an add processing means is provided for conducting an add processing of the output from the plurality of photoelectric converters.
According to the second aspect of the present invention, the detector of the first aspect further comprises a condenser lens, having a sufficiently large aperture with respect to the particle detecting region, placed between the particle detecting region and the plurality of photoelectric converters, wherein the particle detecting region is adjusted to the focal point of the condenser lens, the scattering light due to particles is collimated with the condenser lens, and the collimated light is made incident upon the plurality of photoelectric converters.
According to the third aspect of the present invention, the detector of the first aspect further comprises a concave mirror, having a sufficiently large aperture with respect to the particle detecting region, placed in the position opposite to the plurality of photoelectric converters with the particle detecting region being therebetween, wherein the particle detecting region is adjusted to the focal point of the concave mirror, the scattering light due to particles is collimated with the concave mirror, and the collimated light is made incident upon the plurality of photoelectric converters.
According to the fourth aspect of the present invention, in the above-mentioned detector, the plurality of photoelectric converters are arranged in a linear or plane alignment.